The overall objective is to know the molecular and pharmacological properties of the ionic channel associated with the nicotinic acetylcholine (ACh) receptor and to understand the mechanism by which this receptor and its channel activate and control ionic permeability across the postsynaptic membrane. The ionic channel site has been identified in vitro in Torpedo electric organ membranes by means of its specific binding of (3H) perhydrohistrionicotoxin (3H)H12-HTX), a compound which has been demonstrated electrophysiologically to interact with the ionic channel in skeletal muscle endplates. This binding was inhibited only by ionic channel drugs. We discovered that phencyclidin bound to the ionic channel sites, while nereistoxin bound to the ACh-receptor sites. The initial rate of (3H)H12-HTX binding to the ionic channel of the ACH-receptor was accelerated over 1000-fold in the presence of carbamylcholine, and there was good correspondence between the degree of this acceleration and the stimulation of 22Na ion influx over a wide range of carbamylcholine concentrations (up to 100 mu M). Preincubation of Torpedo membranes with carbamylcholine decreased the initial rate of (3H)H12-HTX abinding, as well as the rate of 22Na ion influx, which may reflect desensitization of the ACh-receptor. d-Tubocurarine (dTC) inhibited the agonist-mediated acceleration of (3H)H12-HTX binding and 22Na ion influx. These findings suggested that H12-HTX bound to closed and open ionic channels with a preference for the latter conformation. It also suggested that the conformational changes associated with activation or desensitization of the receptor can be monitored by studying binding of (3H)H12-HTX to the ionic channel sites as well as by the ACh-receptor-regulated 22Na ion influx. The influence of temperature, cations and pH on (3H)H12-HTX binding suggested that hydrogen bonding and ionic interactions were relatively unimportant to (3H)H12-HTX binding. Thermodynamic studies indicated large positive entropy and enthalpy changes associated with (3H)H12-HTX binding. An ionizable carboxylic group was important to both ACh as well as (3H)H12-HTX binding sites. Sulfhydryl, tyrosine hydroxyl and amino groups were also involved in the latter sites.